Polishing means and method



A g- 12, 1 R. s. om. 2,606,405

PQLISHING MEANS AND METHOD Filed Aug. 17, 1946 3 Sheets-Sheet 1 /&

I FIG INI/EN TOR R 5. OHL

- ATTORNEY Aug. 12, 1952 R. s. OHL

POLISHING MEANS AND METHOD 3 Sheets-Sheet 2 Filed Aug. 17, 1946 /N 5 NTOR R. 5. 0/14 BY M63? 6 /(M ATTORNEY Aug. 12, 1952 R. s. OHL

2,606,405 POLISHING MEANS AND METHOD Filed Aug. 17, 1946 s sheets s heets as as IN VE N TOR RSOHL 8V 116w? GKM A T'TORNEV Patented Aug. 12, 1952POLISHING ltIEANS AND METHOD Russell S. Ohl, Red Bank, N. J., assignorto Bell Telephone Laboratories, Incorporated, New York, N. Y.,acorporation of New York Application August 17, 1946, Serial No. 691,34613 Claims. (c1. 51-124) I This invention relates to method and means ifor polishing brittle crystalline material.

An object of the inventionis to provide an improved device for polishingbrittle crystalline material.

Another object is to provide an improved method of producingsubstantially optically flat surfaces on brittle crystalline material.

A further object is to provide an improved polishing technique forproducing fiat surfaces on pieces of highly pure solidified fusedsilicon.

In an example of practice an approximately flat surface of a slab ofsilicon cut from an ingot of solidified fused silicon of high purity ispolished to produce a substantially'optically'fiat surface. The methodemployed comprises producing relative movements 'of rotation andtranslation between a. tin lap and the surface of the slab of siliconover a nonrepetitive pathwhile the lap and slab are both im'rnersed inwater carrying an abrasive such as sapphirine and dioctyl sodiumsulfosuccinate which is a water soluble chemically inactive detergent.The surface of the tin lap is provided with a spiral groove, thefunction of which is to cause a continual flow of abrasive charged waterfrom the center of the lap toward'the periphery to continually carryaway the particles of silicon removed from the surface of the slab andbring in fresh abrasive. In carrying out the method a rotatable uprightcup mounted'on a vertical shaft is provided with a tin lap having a fiatground surface which has a spiral groove concentric With the shaft. Thetin lap'is centrally positioned in the bottom of the cup with the groundsurface facing upward. A metal disc with the slab of silicon cemented toone face thereof rests on the grooved surface of the tin lap with theexposed surface of the slab in contact with the lap. By means of asecond ro tatab-le shaft, the lower end of which rests in a recess inthe metal disc, the silicon slab is given a reciprocal motion oftranslation across the lap and a motion of rotation about a verticalaxis. The cup is filled with a liquid toa depth sufiicient to immerseboth the silicon'slab and the lap while the cup and lap are rotatingduring the polishing operation. The liquid bath comprises distilledwater containing levigated sapphirine, 3 per cent dioctyl sodiumsulfosuccinate solution and A; per cent potassium bichro-mate. Thepressure for polishing is about pounds per square centimeter, Thepolishing is accomplished by a true abrasive action which is effectivebecause the sapphirine particles are of the Water extremely small and ofthe proper hardness to produce on silicon a highly polishedsubstantially opticallyflat surface. The combination of the 'dioctylsodium sulfosuccinate with the very fine polishing material reduces thesurface tension and increases the effectiveness of the abrasive.

An advantageof this invention is that a substantially optically flatsurface is produced on the slab instead of a lemon peel effect which isproduced by a bufiing method of polishing. Another advantage is that thesurface being polished faces downward which facilitates removal of thewaste particles of silicon disengaged from the surface during thepolishing operation.

-By the method and means of this invention silicon slabs can be polishedto a flatness of at least onequarter wavelength of green light which isconsiderably better than can be obtained by any prior" method known toapplicant.

A more detailed description of this invention will now be given havingreference to the accompanying drawings.

Fig. 1 is a side view of a polishing machine embodying this invention.

Fig. 2 is a plan view of the machine of Fig. 1.

Fig. 3 is a' fragmentary view in section of the 7 machine of 1 along theline 3-3 looking in the direction of the arrows.

Fig. 4 is a vertical section showing the liquid containing rotatable cupwith the grooved'lap and the iron disc with a silicon slab cementedthereto and resting on the grooved lap.

Fig. 5 is a horizontal section of the device of Fig. 4 taken along theline 55 looking in the direction of the arrows.

The same reference characteristics are used to indicate identicalelements in the several figures of the drawings.

Referring now to the drawings, particularly to Fig. 1, a specificembodiment of this invention is shown mounted on a wooden table 5, afragment only: of which is shown. -A rotatable cup 6 is supported on theupper end of a shaft 1 which carries "a rigidly attached tapered cap.The bottom portion 8 of the cup 6 is provided with a tapered recesswithin which the tapered cap on the shaft 1 fits snugly. A threadedcollar 9 on a threaded cylindrical portion of the tapered cap on the,shaft 1 facilitates removal of the cup 6, the cup 6 being loosened byscrewing the a belt I3 on the output pulley M of a speedreducingmechanism I5. Power for the mechanism I5 is furnished by a motor-drivenshaft l6 (see Fig. 2) through an all-speed transmission mechanism H andbelts I8 and I9. Mechanism l1 comprises an input pulley 20, an outputpulley 2| and a speed control handle 22. The speedreducing mechanism I5is driven by belt IS on input pulley 23. Belt I8 is driven by, pulley 24on power shaft [5.

Movements of oscillation and rotation are imparted to the material to bepolished by the mechanism now to be described. Referring first to Fig.4, a slab 30 of material to be polished is cemented to the face 31 of ametal disc 32, such as steel or brass. The disc 32 is adapted to bemoved horizontally by shaft 33 through the intermediary of pin 34 whichis inserted into a hole in the lower end of shaft 33 and rigidly securedto the shaft 33 by aset screw. The lower end of pin 34 projects into ahole in disc 32. A washer 35 surrounds the pin 34 between the shaft 33and the disc 32. A relatively large steel washer 36 surrounds acylindrical projection 25 of the disc 32 with a turning fit.

Referring again to Fig. 1 the shaft 33 is carried by an arm 31 pivotedbetween the legs of a U-shape member 3i! (see also Fig. 3) which arm isadapted to oscillate about a vertical axis and is driven by an eccentriccam 39 acting on a cam follower 43. The'eccentric cam 39 is secured tothe upper end of shaft 4| which is supported in a housed bearing 42bolted to thetable 5. The shaft 4| is driven from a pulley on the lowerend of shaft '1 through an .all-speedtransrnission mechanism 43 andbelts 44 and 45. The

mechanism 43. comprises an input pulley'46, an

output pulley 41 and a speed control handle 48. The shaft 41 is drivenby belt 45 on pulley 49. Input pulley 46 is driven by belt-44 on pulley50 (see Fig. 2) on the lower end of shaft 1.

In addition to the elements previously mentioned, the mechanism foroscillating thematerial 30 to be polished comprises a 'U-shaped support5|, the legs of which are bolted to the table 5. The U-shaped member 3.3rests on the upper surface of the cross member of LJ-shaped member 5|and is adapted to rotate about a vertical axis. A downwardly extendingsteel rod having a drive fit in the cross member of U-shaped member 33fits into a downwardly extending steel bushing 52 having a drive fit inthe cross member of U-shaped member 5| and being welded to the crossmember. This bushing 52 serves'as a bearing for the downwardly extendingrod of U-shaped member 33 to support U-shaped member 38 for oscillationabout a vertical axis on top of U-shaped member 5!. Oscillating arm 31is supported between the upstanding legs of U-shaped member 38 by pivots53. Shaft 33 is supported in a bearing 54 carried by arm 3'! at the endopposite to the pivots 53. As cam 39 rotates cam follower 4D isoscillated about the axis of bushing 52 to oscillate the U-shaped member38 and so to oscillate the arm 37 carrying the shaft 33 and the material30 to be polished.

The rotation of shaft 4| also rotates shaft 33 through the intermediaryof belts 55 and 56. Belt 55 is driven by pulley 57 secured to the shaft4| just above bearing 42 and drives pulley 58. Pulley 58 drives pulley53 through the intermediary of shaft 69, to which shaftboth pulleys 58and 53 are rigidly secured. Belt 55 is driven by pulley 59 and drivespulley SI which is rigidly secured to the top end of shaft 33. Shaft 60is supported in bearings in the legs of U-shaped member 62. The lowerpart of the cross member of the U-shaped member 62 is welded to a crossbar 63, the ends of which crossbar are welded to the upper portion ofthe legs and the cross member of U-shaped member 5! to support shaft 50with its axis vertical.

Referring now to Figs. 4 and 5, the arrangement of the tin lap withinthe rotatable cup r is raised by rotation about the pivot 53.

6 is shown in more detail. Ihe lap 13 consists of substantially pure tinin the shape of a broad faced washer, which washer fits snugly into afinished recess H in the bottom of the cup 3. One face of this washer 0rlap 70 which is the upper face after the lap is assembled within the cup'5, is provided vitha spiral groove 12 of many turns starting at thehole in the center and ending at the periphery of the washer 13. Thegroove '12 is V-shaped 0.005 inch deep with a (SO-degree angle betweenthe sides and hasa pitch of 3 inch. A few only of the many turns ofgroove 12 are shown in Figs. 4 and 5.

The power from the driving motor is transmitted to the various drivenelements of the device as described hereinbefore and in the directionindicated by the arrows in Fig. 2.

The relationship of the slab 3G to be polished and the lap 15 is shownin Fig. 4'. The polishing is'done under water carrying the abrasive.When the device is operating, the cup 6 and lap 10 are rotating aboutthe axis of shaft 1 and the surface of the liquid in the cup 6 assumesthe approximate shape shown by the lines 13. In a modified arrangementthe liquid is pushed back toward the center by a piece of s: inch leadsheet fastened to a support on the table and overhanging into the outerperiphery of the liquid. This permits less liquid to be used andthepoiishing disc can be run faster. The oscillating arm 3'. is in theposition shown in Fig. 1. The opening in the arm 37 through which thestationary U-shaped member E2 and shaft 60 protrude as shown in Fig. 2,is large enough to permit the arm 31 to oscillate so that the slab 30 ismoved substantially radially across one side of the lap 19. The centerof the pulley 59 is close-enough to the axis of oscillation of the arm-37 so that the tension on belt 56 is not changed materially fordifferent positions of the arm 31 during each oscillation. In order toremove the slab from the machine the arm 31 This can be done withoutremoving the belt 56 which is merely loosened as the arm 3'! is raised.The speed of rotation of the lap 10 is adjusted by means of theall-speed transmission mechanism vI! and the relative speeds of rotationof the lap 10 and the shaft 33 is controlled by the all-speedtransmission mechanism 43.

This. polishing device has been found to be particularly useful inpolishing slabs of solidified fused silicon of high purity. Suchmaterial is produced by fusing silicon powder of high purity in a silicacrucible in an electric furnace and allowing the fused silicon to coolat a controlled rate. Such material has the scratch hardness ofcrystalline quartz but is not nearly as strong. Itv cannot be machinedto approximate shape like malleable metals. It has a very small elasticrange and therefore will stand very little bending force withoutbreaking. The usual buffing methods of applying a final polish resultonly in the. cutting tool.

surface on slabs ofsuch material.

For a detailed description of amethod of producing material which can bepolished to advantage by the method and means of this inventionreference'is made to Patent 2,402,582 of J. H.

Scalf issued June '25, 1946 on application Serial No. 386,835filed'April 4 1941. I

.- The process .of polishing a slab-of siliconwill now be described. a:

A slab of silicon is out from a suitably sized and homogeneous piece .ofsilicon. 1 A Norton metal bonded slicing Wheel is most suitable for thispurpose. The peripheral speed of. the slicing wheel may be conveniently.aboutl'zofeet per minute and should be so arranged. as to be constantlyWashed in a sludge accumulator containing otherwise clean water or inasmall stream of clean water." Such slab is conveniently about 0.028inch thick. This slab may then be flattened on both sides by lapping iton a flat cast iron lap using American OpticaliCompany abrasive M 302.The flattened facesare approximately parallel and the slab is 'now about0.019 inch thick. One flattenedi'surface may then'be cemented to the endof a round metal form with. lapidarists cement (unpolymerized' shellac).The silicon slab can now be rounded by cutting with they diamond slicingwheel, or if more accuracy is required, the final roundness may begotten by grinding in a lathe with a con- ,tinuously: moistenedresinoid-bonded diamond wheel having a mesh of about 400 or finer toinsure the absence of' deep 'chippingialon'g the edges. The slabjmay nowbe removed from the .rounding form and mounted on the finalpolishineiig.

An optically fiat productis readily obtained by closely following thenow-to-be. described procedure. A cast iron lap is prepared by cutting aflat surface on a good lathe. The surface should not be free fromcutting marks leftby ridges which permit the abrasive to out under thewhole surface of the; slab.. When the slab has been-broughtvto'afiat'matt.finish with M 302 abrasive mixed: with distilled water,about per cent dioctyl sodium sulfosuccinate so lution andzaboutiper.cent potassium Y bichromate it"is ready'f'or the polishing procedure.

under running water, the slab is placedin the first polishing cup. Thiscup may contain a .pure tin lap Whose surface has been machined smoothand 60 degree grooves 0.005;inch deep cut into the surface at a pitch of,inch. A small quantity of 400 mesh boron carbide is contained in theliquid which covers the lap. The liquid is made up of distilled watercontaining about percentdioctyl sodium sulfosuccinate solution and"about percent potassium bichromate A fine diamond charged tin lap worksfaster for the first cup'than the 400 mesh boron carbide; The pressureis about ten pounds. The machine is now operated until all the pits fromthe previous matt finish are removed. Depending on the size of thesilicon slab, the timerequired for this may vary from about 3 to 30minutes.

This procedure is repeated with levigated 800 mesh boron carbide untilthe coarsewscratches left by the previous operation are removed.

Such cutting marks indicate timeter.

. ranged that the sample relation to the rotation of which are removed.in the use of diamantine (orits equivaentlpolish powder in place ofsapphirine. p,

A most important feature of this polishing technique is the. machine forproducing 'thedesired-mechanical movements. The machine rotates thecutting laps. Due to frictional differences depending ,upon the distancefrom. the center of the lap, a rotating torque. is developed in thesample. In-somecases this torqueis-suflicient to spin the sample beingpolished in other casesit is not, therefore the rotationis aided byapower drive. In addition, 'a reciprocating .motion is given the sampleso thatthe-lap does not become Worn and develop uneven lgrooves. Thisreciprocating motion is controlled byfa stroke speed adjustment.Thespeeds are'so arhas no fixed multiple the lap so as to avoidtheformation of. a pattern onthe lap-which would be harmful. Y, i

The polishing process is carried outunder water-and is strictlya cuttingprocess as compared. to a buiiing process. .The piecesof silicon finalprocess are so fine that a chemical-action with Water. seems to occur.This isindicated by the. formation of a surface layer as indicatedbyelectr ical tests. This surface layer gives everyindication of beingortho-silicic acid produced by theaction of the finely dividedsiliconupon the. water. The

.ortho-silicic acid surface .will absorb and lose water until it retainsless than 17 'per cent water cent of 47 per cent hydrofluoric aci'dadded. I An immersion of one I second immediately followed by a wash inrunning distilledlwater removes the ortho-silicic acid Without affectingthe silicon surface. Sometimes the meta-rsilicic acid surfaces requireas much as 10-second treatment if they have been thoroughly dehydrated.

An excellent'polish can also be obtained by the use of pure lead laps.'Lead used for this purpose should be free from silver and other hardmetals, although. tinin various proportions does littleharm]. If'leadlaps are used; a few drops of water glass must be substituted in placeof potassium bichromate whichlatter chemical attacks the lead in time.

1 The purpose of the dioctyl sodium sulfosuccihate is to reduce theliquid surface tension which allows the fine abrasive to cut morereadily.

The potassium bichromate is added to arrest corosion of themetal partswithin the lapping cups, It is especially effective in the prevention ofrusting of cast iron. The chromate is not ordinarily used with lead lapsbecause of the which they come in contact during the polishing process.Examples of such materials are:

Dioctyl sodium sulfosuccinate Dihexyl sodium sulfosuccinate Diamylsodium sulfosuccinate Dibutyl sodium sulfosuccinate Isopropylnaphthalene sodium sulfcnate The commercial product of American'Cyanamid and Chemical Corporation which contains per cent of puredioctyl sodium sulfosuccinate, a mutual solvent consisting of one of thecompounds selected from the group of compounds consisting of alcohol,acetone, ethylene glycol ,monoethyl ether and diethylene glycol monoas asubstitute for potassium bichromate when "a lead lap is used, alsofunctions as a wetting agent for the practice of this invention.

The tin lap may be charged with sapphirine togood advantage. To effectsuch charging the grooved surface of the lap is coated with a paste ofsapphirine and water and the coated surface is rolled with a metalroller so that particles of the sapphirine are embedded in the surfaceof the lap. The charged lap is used for polishingjin the same manner asan uncharged lap in the manner described hereinbefore.

A convenient size of silicon slab is about one and one-eighth inchessquare polished on a lap four and five-eighth inches in diameter.

The same polishing technique has been used successfully on germanium,zincite, hardened steel and a number of other solids to produce anoptical finish.

The invention as defined in theappended claims may find embodiment instructures and methods differing widely from the specific structures andmethods described. in this specification. Suchv embodiments come withinthe purview of the claims.

What is claimed is:

1. The method of polishing a crystalline material of the nature ofhighly pure crystalline silicon with respect to the action of abrasivesthereon comprising moving a surface of said material to be polished incontact with the surface of tin lap in a cyclically nonrepetitive path,immersing the material during movement in a liquid bath of distilledwater containing a finely divided abrasive and dioctyl sodiumsulfosuccinate, and rotating 2. container within which said lap isfitted snugly, said container containing said liquid during the movementof said material for polishing said surface.

2. The method of polishing solidified fused crystalline silicon whichcomprises immersing a surface of a tin lap and a silicon surface to bepolished in distilled water carrying finely divided sapphirine andsurface active agent which is chemically inactive with respect tothematerial in contact with said water, rotating the tin lap, andmoving. thesurface of the silicon body to be polished in contact withthe surface of and repeatedly transversely of the rotating tin lapsurface while both are immersed in said water.

3. The method of polishing a crystalline material of the nature ofhighly pure crystalline silicon with respect to the action of abrasivesthereon comprising moving a surface of said material to be polished incontact with the'surface of a tin lap in a cyclically nonrepetitive pathwith the surface of the' material being polished facing downward,immersing the material during movement in a liquid bath of distilledwater containing finely divided sapphirine and isopropyl naphthalenesodium sulfonate, and rotating a container containing said lap and saidliquid during the movement of said material for polishing said surface-4. The method of polishing a crystalline material of the nature ofhighly pure crystalline silicon with respect to the action of abrasivesthereon comprising moving a surface of said material to be polished incontact with the surface of a soft metallic lap in a cyclicallynonrepetitive path with the surface of the material being polishedfacing downward, immersing the material during movement in a liquid bathof distilled water containing a finely divided abrasive, dialkyl sodium'sulfosuccinate and potassium 'bichromate, and rotating a containerwithin which said lap is fitted snugly, said container containing saidliquid during the movement of said material for polishing said surface.

5. The method of polishing solidified fused crystalline silicon whichcomprises immersing the surface of said silicon to be polished indistilled water carrying finely divided sapphirine and dialkyl sodiumsulfosuccinate and in contact with a metallic surface having hardness ofthe order of that of tin, and maintaining the contacting surfacesimmersed while giving them relative movement to effect the polishingaction.

6. The method of polishing a crystalline material of the nature ofhighly pure solid crystalline silicon with respect to the action ofabrasives thereon comprising moving a surface 'of said material to bepolished in contact with water containing a finely divided abrasive anda dialkyl sodium sulfosuccinate, and rotating a container within whichsaid lap is fitted snugly, said container containing said liquid duringthe movement of said material for polishing said surface.

7. The method of polishing a crystalline material of the nature ofhighly pure solid crystalline silicon with respect to the action ofabrasives thereon comprising moving a surface of said material to bepolished in contact with the surface of a soft metallic lap in acyclically non-repetitive path, immersing the material during movementin a liquid bath of distilled water containing a finely divided abrasiveand dioctyl sodium sulfosuccinate, and rotating a container within whichsaid lap is fitted snug-1y, said container containing said liquid duringthe movement of said material for polishing said surface.

8. The method of polishing a crystalline material of the nature ofhighly pure solid crystalline silicon with respect to the action ofabrasives' thereon comprising moving a surface of said materia1 to bepolished in contact withthe surface of a soft metallic lap in acyclically nonrepetitive path, immersing the material during movement ina liquid bath of distilled Water containing a finely divided abrasiveand isopropyl naphthalene sodium sulfonate, and rotating a containerWithin which said lap is fitted snugly, said container containing saidliquid,

which said lap is fitted snugly, said container containing said liquidduring the movement of said material for polishing said surface.

10. A machine for polishing highly pure crystalline silicon comprisingan upright cupshaped member adapted to be rotated about its axis in avertical position, a cylindrical tin lap lying flat on the bottom ofsaid member adapted to be rotated thereby, a liquid within said cup incontact with and covering said lap, said liquid comprising distilledwater, levigated sapphirine, a small percentage of dioctyl sodiumsulfosuccinate and potassium bichromate immersing said lap duringrotation, a crystal holder positioned in opposing juxtaposition to thelap and adapted to hold a slab of highly pure crystalline silicon, meanscoupled to one of said abovementioned opposed elements for urging themtoward one another, and means secured to said holder and coupled withsaid cup-shaped member for moving said holder repeatedly transversely ofthe direction of movement of said lap as said lap is rotated.

11. A machine for polishing highly pure crystalline silicon comprisingan upright cupshaped member adapted to be rotated about its axis in avertical position, a cylindrical tin lap lying flat on the bottom ofsaid member adapted to be rotated thereby, a liquid within said cup incontact with and coverning said lap, said liquid comprising distilledwater, levigated sapphirine, a small percentage of isopropyl naphthalenesodium sulfonate and potassium bichromate immersing said lap duringrotation, a crystal holder positioned in opposing juxtaposition to thelap and adapted to hold a slab of highly pure crystalline silicon, meanscoupled to one of said above-mentioned opposed elements for urging themtoward one another, and means secured to said holder and coupled withsaid cup-shaped member for moving said holder repeatedly transversely ofthe direction of movement of said lap as said lap is rotated.

12. A machine for polishing highly pure crystalline silicon comprisingan upright cup-shaped member adapted to be rotated about its axis in avertical position, a cylindrical tin lap lying fiat on the bottom ofsaid member adapted to be rotated thereby, a liquid within said cup incontact with and covering said lap, said liquid comprising distilledwater, levigated sapphirine, a small percentage of dialkyl sodiumsulfosuccinate and potassium bichromate immersing said lap duringrotation, a crystal holder positioned in opposing juxtaposition to thelap and adapted to hold a slab of highly pure crystalline silicon, meansin operative engagement with one of said above-mentioned opposedelements for urging them toward one another, and means secured to saidholder and coupled with said cup-shaped member for moving said holderrepeatedly transversely of the direction of movement of said lap as saidlap is rotated.

13. A machine for polishing highly pure crystalline silicon comprisingan upright cup-shaped member adapted to be rotated about its axis in avertical position, a cylindrical substantially pure lead lap lying flaton the bottom of said member adapted to be rotated thereby, a liquidWithin said cup in contact with and covering said lap, said liquidcomprising distilled water, levigated sapphirine, a small percentage ofdioctyl sodium sulfosuccinate and Water glass immersing said lap duringrotation, a crystal holder positioned in opposing juxtaposition to thelap and adapted to hold a slab of highly pure crystalline silicon, meansin operative engagement with one of said above-mentioned opposedelements for urging them toward one another, and mean secured to saidholder and coupled with said cup-shaped member for moving said holderrepeatedly transversely of the direction of movement of said lap as saidlap is rotated.

- RUSSELL S. Ol-IL.

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